1. Field of the Invention
The present invention relates to a sputtering method such as a magnetron sputtering method, which is one technique for forming thin films.
2. Description of Prior Art
The sputtering method is a method whereby a thin film is formed by generating a plasma, typically by creating a gaseous discharge in a low-vacuum atmosphere, making positive ions of the plasma collide with a target arranged on a negative electrode called a sputtering electrode, and thereby depositing sputtered particles produced by the collision on the substrate. This sputtering method is widely used in film deposition steps since it enables composition to be controlled and operation of the apparatus is comparatively simple.
FIG. 5 shows the layout of a prior art magnetron sputtering apparatus. In FIG. 5, a vacuum chamber 101 has an evacuation port 102 from which vacuum chamber 101 is evacuated by a vacuum pump (not shown). The evacuation port 102 has a main valve 104, and a moveable valve 103 whereby the evacuation conductance can be adjusted. The vacuum chamber 101 further has a gas introduction pipe 105 and a gas flow rate controller 106 mounted on gas introduction pipe 105. Electrical discharge gas 107 is introduced into vacuum chamber 101 from gas introduction pipe 105; usually, argon gas is employed. 108 is a gas introduction valve.
A target 109 is held on a sputtering electrode 110, that is connected to a discharge power source 111 and is provided with a magnet 112 arranged at the back face of target 109. A substrate 114 on which a thin film is formed is placed on a substrate holder 113 opposite the substrate electrode 110. An insulator 116 is mounted so as to isolate sputtering electrode 110 from vacuum chamber 101.
Operation of the sputtering apparatus constructed as above will now be described. First of all, the interior of vacuum chamber 101 is evacuated to about 10.sup.-7 Torr by means of a vacuum pump from evacuation port 102. Next, discharge gas 107 is introduced into vacuum chamber 101 through gas introduction pipe 105 that is connected to one end of vacuum chamber 101, while the pressure within vacuum chamber 101 is maintained at about 10.sup.-3 to 10.sup.-2 Torr. When negative voltage or high-frequency voltage is applied by DC or high-frequency discharge power source 111 to sputtering electrode 110 on which target 109 is mounted, by the action of the electric field produced by discharge power source 111 and the magnetic field produced by magnet 112 arranged at the back face of target 109, a ring-shaped plasma is produced by the discharge in the vicinity of the front face of target 109, giving rise to the sputtering phenomenon. The thin film is thus formed on substrate 114 arranged on substrate holder 113 by the sputtered particles that are discharged from target 109.
However, in the conventional arrangement described above, since gas introduction into vacuum chamber 101 and evacuation from vacuum chamber 101 are both performed during the formation of the thin film, the pressure on the front face of target 109 becomes uneven. As a result, the distribution of sputtered particles discharged from target 109 is uneven, giving rise to the problem that the thickness of the thin film formed on substrate 114 cannot be made uniform.